This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 竴 is uncertain. It visually combines 立 (stand) and 尊 (revere), but the original semantic and phonetic roles are unclear.
With no clear meaning, a mnemonic is not possible. The shape shows 立 (stand) beside 尊 (revere), but this does not reliably point to a meaning.
For シュン, imagine a person named Shun standing (立) respectfully before someone they revere (尊): Shun -> シュン.